The present invention relates to an electric power steering system which assists steering by virtue of a rotating force of a motor which is controlled to be driven based on a result of a detection of a steering torque applied to a steering member.
In an electric power steering system in which a steering assist motor is driven in response to a rotating operation of a steering member such as a steering wheel, so that a rotating force of the motor is applied to a steering mechanism so as to assist steering, a steering torque that is applied to the steering member needs to be detected for use to control the drive of the steering assist motor, and conventionally, a torque sensor, which is provided at an intermediate position of a steering shaft which connects the steering member and the steering mechanism together, is used to detect the torque.
With the electric power steering system like this, there exists a problem that when the torque sensor is in a failed state, the application of a steering assist force by the motor which is controlled to be driven based an erroneous detected torque value detected by the failed torque sensor makes steering unstable. To cope with this, conventionally, two torque sensors having the same configuration are provided in parallel at the intermediate position of the steering shaft, and a detected torque value detected by one torque sensor (a main torque sensor) is used to control the drive of the steering assist motor, whereas a detected torque value detected by the other torque sensor (a sub-torque sensor) is used to determine on the failure of the main torque sensor, whereby when a difference between the detected torque values detected by both the torque sensors exceeds a preset threshold value, the main torque sensor is determined to be in a failed state, and the control of the drive of the steering assist motor (the assist control) is prohibited to thereby prevent the occurrence of unstable steering.
While the aforesaid failure determination is performed accurately when the detected torque values detected by the main and sub-torque sensors coincide relative to the same torque input, both the torque sensors inevitably suffer from a difference in characteristics which is attributed, for example, to dimension errors of constituent components, positioning errors at the time of assembly and the like. To cope with this, conventionally, after assembly, a predetermined torque is experimentally applied so as to obtain detected torque values by main and sub-torque sensors, and a difference between the two detected torque values is used as a threshold value for use in the failure determination so as to offset the characteristics difference to thereby attempt to increase the accuracy of the failure determination (refer, for example, to Patent Document No. 1).
Patent Document No. 1: JP-A-10-324254
In the assembly line where the above test is carried out, however, there exist many disturbance factors such as various types of noise which will affect detection, and hence, there exists a risk that the setting of a threshold value is erroneously carried out based on detected torque values which are obtained in the above environment, which deteriorates, on the contrary to the intention, the accuracy of a failure determination made based on the threshold value so set, whereby an erroneous assist control is implemented to trigger a reduction in steering stability.
In addition, the threshold value which is set as has been described above is generally written to be stored in a memory IC such as EEPROM which is provided in an assist controlling ECU and is read out of the ECU for use every time the ECU is activated in response to an ON operation of the ignition key. However, there may occur an error in writing or reading out the threshold value, and in this case, too, a failure determination is carried out based on the erroneous threshold value, leading to a risk that the reduction in steering stability is triggered.